The semi-permeable membranes that are useful for water desalting by reverse osmosis are swollen (water retaining) polymer films which appear to be in a poreless phase in the film matrix and in which the salt and water solution components to be separated dissolve independently and move across by diffusion. The driving force for water transport is that part of the working pressure imposed upon the feed water (salt water) which exceeds the osmotic pressure of the feed water, while the driving force for salt transport is the salt concentration of the adjacent feed water itself.
Asymmetrical semi-permeable membranes, frequently described in technical and patent literature, consist of an extremely thin, practically nonporous surface film or skin over a sponge-like porous carrier sheet that constitutes the major thickness of the membrane. In the production of an integral-asymmetrical membrane, its cover sheet or skin, which constitutes the semi-permeable membrane proper, is developed from one and the same polymer material as the carrier sheet. The carrier sheet, being porous, does not contribute significantly to desalting (see West German Pat. No. 2,552,282).
The capacity for rejecting salt is a characteristic property of the polymeric membrane material employed. It is almost independent of the thickness of the membrane. The product water flow depends upon the capacity of the polymeric membrane material to incorporate bound water (hydrophilism). The product water flow increases approximately linearly with decreasing sheet thickness (membrane thickness) and with increasing pressure difference across the membrane. This imposes a requirement for the thinnest possible membrane that is nevertheless capable of sustaining the working pressure.
The conventional procedure for producing integral-asymmetrical membranes, for example with typical secondary cellulose acetate for the entire membrane structure, comprises the steps of: (a) forming and homogenizing a casting solution consisting essentially of polymer, solvent medium and swelling medium; (b) spreading out the casting solution as a film that is typically of 0.3 mm. thickness; (c) permitting a portion of the solvent medium to evaporate to the air to preliminarily form the cover sheet; (d) immersing the film in a cold water bath to cause the non-polymeric components to be dissolved out while, during concurrent gelling, water is taken up by the membrane; and (e) heating or annealing the swollen membrane in a warm water bath, wherein the membrane solidifies itself in the previously established structure with a relatively small but significant reduction in its water content.
At every point, this process is controlled with a view to the permeability characteristics of the resulting membrane, so that the most important casting variables--next to the selection of the polymer material itself--are the period of evaporation of the freshly spread film in the air, and the temperature of the concluding warm bath annealing treatment.
A typical heretofore conventional casting solution consists of 25% by weight of secondary cellulose acetate (cellulose diacetate) as the membrane component proper, 45% by weight of acetone as the solvent medium, and 30% by weight of formamide as a water-soluble solution swelling medium (U.S. Pat. No. 3,344,214).
For one-step desalting by reverse osmosis of strongly salty water such as sea water, several special requirements are imposed upon the membranes. The polymeric materials employed for such membranes must have a characteristic salt rejection capability that is high enough to enable drinking water quality to be attained on the product water side of the process. This requires a salt rejection of more than about 99%, for salt depletion of the feed water from over 3.5% total salt content to less than 0.05%.
Such salt rejection capability must be realized in a membrane structure that has the thinnest possible cover sheet, so that a product water flow can be attained that is adequate for practical applications. The desalting cover sheet, moreover, must be exceptionally free from such defects as pores and inhomogeneities that would permit convective passage of unaltered feed water through it. This requirement increases in importance with increasing salt content of the water to be treated. In addition, desalting of sea water, as compared to the treatment of merely brackish water, requires the membrane to have an increased resistance to compacting under pressure, because mechanical compacting of the membrane tends to decrease the yield of product water. Compacting of the membrane is particularly a problem with the desalting of sea water because the required working pressure of the feed water is governed by its osmotic pressure and must therefore be higher for sea water desalting than for treatment of brackish water. Thus, along with adequate salt rejection and adequate flow capacity, the best possible flow stability should also be available from a membrane employed for sea water desalting, that is, its rate of production of desalted water should not decrease excessively during a long period of subjection to operating pressure.
These requirements have not been satisfied by integral-asymmetrical membranes of cellulose diacetate produced according to the heretofore conventional method outlined above. It is true that they typically provide good flow capacities when used for the desalting of brackish water, but they are nevertheless unsuitable for the desalting of sea water, particularly in relation to flow stability and capacity for restraining passage of salt.
It is known that the inherent salt restraining capacity of cellulose acetate increases with increasing degree of acetylization, up to the threshold of fully acetylated cellulose triacetate; but this increase in salt restraining capability is accompanied by an increasing and undesired densification towards water as well as salt. A cellulose acetate of natural origin with an increased degree of acetylization behaves like a mixture of cellulose diacetate and cellulose triacetate in consequence of production-conditioned statistical variations in the distribution of the acetyl groups in the polymer chains. It has therefore been proposed that gradually increased adjustments of acetate content be obtained by employment of corresponding mixtures of two cellulose acetates (U.S. Pat. No. 3,497,072). A substantial difficulty with this procedure lies in the markedly differing solubility characteristics of the two cellulose acetate types in the organic solution media that are suitable for membrane production, which, in the end, prevent the expected benefits of the cellulose triacetate portion from being realized in full measure and without disadvantageous side effects. Thus, for example, cellulose triacetate is insoluble in acetone, which is a preferred solvent medium, whereas cellulose diacetates are explicitly classified as acetone-soluble acetates. With the inclusion of further solvent media, particularly dioxane, it becomes possible to prepare homogeneous casting solutions with both of these cellulose acetates, but the viscosity of such a casting solution rises so abruptly with increasing proportions of cellulose triacetate that even with the employment of short-chain cellulose triacetate it becomes practically impossible to ensure that the casting procedure (step (b) in the foregoing example) will result in a film that is free from defects. The most suitable viscosity range for the casting solution lies between 5,000 and 7,000 cP with a minimal concentration of 12% by weight of membrane forming polymer; above a viscosity of, at most, 10,000 cP, reproducible membrane production is not possible.
To attempt to avoid defects by producing a heavier cover sheet, as by lengthening the evaporation time (step (c)), obviously entails a sacrifice of permeability to product water and, in addition, increase the danger of demixing of the two polymer components. The tendency towards demixing exists most markedly in the immersion step of the production process (step (d)), in which gelling of the membrane takes place while the swelling medium and the solvent medium are exchanged for water. When this exchange proceeds at different speeds for the several components of the casting solution, the mixture undergoes a premature precipitation of the heavier soluble cellulose triacetates, with the result in many cases that the cellulose triacetate is dispersed into isolated areas in a membrane otherwise composed of cellulose diacetate, and the increase in salt rejecting capacity that has been sought is not realized.
There have been previous attempts to employ certain minerals as fillers to improve the long-term flux stability of cellulose acetate membranes. See, for example, the article by I. Goossens and A. Van Haute in Desalination, No. 18 (1976), p. 203. Goosens and Van Haute incorporated from about 3% to over 20% by weight of mineral filler material in otherwise conventional casting solutions. Although they found that certain mineral fillers brought about some improvement in compaction resistance, and hence in long-term flux stability, they also found that the incorporation of such fillers caused a decrease in salt rejection, and therefore the membranes containing them were generally not suitable for sea water desalting, although possibly useful for desalting brackish water. From the standpoint of the present invention it is noteworthy that one of the mineral fillers tested by Goosens and Van Haute was Bentone-38, a hectorite (magnesium silicate) having a coating of an ammonium organic derivative. They observed that this filler seemed to have less adverse effect upon salt rejection than others that they tested, but that "there is a suddenly strong decrease of rejection at lower filler concentrations for these membranes."
The above mentioned West German Pat. No. 2,552,282, published in 1977, discloses an integral-asymmetrical membrane produced from a casting solution into which was incorporated a very small percentage of hydrophilic bentonite (Montmorillonite or Hectorite) that contained exchangeable cations. The casting solution was otherwise generally conventional, consisting of a water-swelling polymer, a solvent medium and a swelling medium. The bentonite was incorporated as a thixotropic agent to improve the castability of the casting solution and the homogeneity of the resulting membrane. The bentonite was first swollen in water, in a mixture of 1 to 5 parts by weight of bentonite to 50 parts by weight of water, and this suspension was then mixed with 50 to 100 parts by weight of the solvent medium (for example, acetone) for the casting solution. The mixture was then added to the casting solution in an amount such that there was 0.1 gram of bentonite in every 100 grams of casting solution. At, for example, a 98% salt rejection there was a substantial improvement in product water flow as compared to a comparable membrane in which the bentonite was not incorporated, but there was only a negligible improvement in flux stability. A membrane in which bentonite was incorporated in accordance with the disclosure of this West German patent could achieve a maximum salt rejection of about 98% at a practical product water flow rate. Although this was somewhat better than an otherwise identical membrane without the bentonite, it was not good enough for sea water desalting, which requires that salt rejection at practical flux rates must be better than 98.9%.
In general, therefore, it can be said that previous attempts to improve any one of the three characteristic properties of reverse osmosis membranes--flux or desalted water production rate, salt rejection capability, and pressure-related flux stability during an extended period of use--have invariably led to a corresponding loss in one or both of the other two. Furthermore, it has not heretofore been known how to produce a reverse osmosis membrane that was capable of producing product water of potable quality from sea water (in distinction to merely brackish water) at practical production rates and with adequate flow stability through a reasonable operating life on the order of thirty days or more.